In the field of radiation detection and imaging it is well known to use so called intensifying screens that contain luminescent material for converting the invisible penetrating radiation into visible light. The light produced by the luminescence of the penetrating radiation in the intensifying screen can then be made available for detection on film or by other electronic light-sensitive detector means such as a CCD (charged coupled device), photocathode or photodiode.
For example, in x-ray imaging applications, the intensifying screen will contain x-ray luminescent material, typically a thin layer of phosphor particles. The particular phosphor composition selected will be determined in accordance with the desired selected x-ray application in order to emit a relatively large number of light photons for each x-ray photon which strikes a phosphor particle in the phosphor layer.
It is desirable that the phosphor layer or intensifying screen exhibit good brightness or light output, good radiation absorption efficiency and good spatial resolution.
For medical x-ray applications in particular, the design of such intensifying screens has involved a trade-off between screens of large thickness, which result in increased luminescent radiation for a given x-ray level, but which also produce decreased image sharpness, and screens of less thickness, which result in improved image sharpness relative to the thicker screens, but which also require more x-ray radiation to produce acceptable film images, thereby increasing the x-ray dosage to which the patient must be exposed. In practice, the thicker or high speed screens are utilized in those applications which do not require maximum image sharpness, thereby reducing the patient exposure to x-rays, while thinner or medium speed and slow speed screens are utilized when increased image resolution is required. These latter screens employ thinner phosphor layers and may incorporate dyes to minimize transverse propagation of light by attenuating such rays more than a normal ray which travels a shorter path. In general, thinner or slow speed screens require approximately 8 times the x-ray dosage of thicker high speed screens.
There are many applications in medical x-ray imaging, industrial x-ray imaging and x-ray crystallography, among other disciplines, where spatial resolution must remain high. Heretofore, good resolution x-ray screens which have a radiation absorption efficiency upwards of 90%-100% simply did not exist, since 100% radiation absorption efficiency for thin phosphor screens have been generally thought impossible to achieve.
In accordance with the known practice, phosphors are coupled directly to radiographic film for use in medical x-ray imaging. It is also well known in the art to deposit a phosphor layer on a fiber optic face plate which, in turn, is directly coupled to a CCD camera. The fiber optic face plate functions as a light guide for channeling the light photons produced in the phosphor layer to the detector of the CCD.
Such electronic devices as CCDs are sensitive to direct penetrating radiation. Direct incidence of penetrating radiation onto CCDs will cause additional noise in the image and over time damage such devices by forming traps that result in reduced charge transfer efficiency and higher dark current.
The use of a phosphor deposited directly onto a CCD, i.e., without any fiber optic device, therefore has very limited applications. The phosphor frequently does not provide the shielding required to protect the CCD from radiation damage and keep the noise low from direct excitation of the radiation onto the CCD. Normally, this results in severe speckle noise which fogs the optical image that is deposited onto the CCD. This, as stated above, severely limits the lifetime of the CCD. In summary, direct imaging onto a CCD is possible, but it is not recommended with the combined use of a phosphor. A phosphor deposited onto a fiber optic device which is bonded to a CCD will provide shielding, but will have a relatively low conversion of radiation into light.
Accordingly, it would be desirable to be able to provide a luminescent device which combines the shielding capabilities of a fiber optic device and the brightness and good spatial resolution capabilities of a thin phosphor screen.
Various patents have proposed techniques for fabricating a phosphor layer which has both good resolution and enhanced brightness. The basic approach taken by the prior art has been directed to methods for depositing the active phosphor particles in an array of cells or pixels which are separated by wall members that are disposed generally parallel to the direction of x-ray travel. See for example, U.S. Pat. No. 5,302,423. The purpose of the wall members is to reflect light emitted by the pixelized phosphor particles and thereby prevent scattered light from reaching the detection means and contributing to a foggy image. Such pixelization techniques, however are complex and expensive and still require additional phosphor layer thickness to ensure a desired level of radiation absorption efficiency.
A radioluminescent or scintillating fiber optic is a special type of fiber optic material which has a core glass containing the rare earth element terbium. When activated by ultraviolet light, x-rays, or ionizing particles, terbium fluoresces in the green and peaks at a wavelength of 550 nm. Therefore, a face plate made of this type of fiber optic material can be used both as a scintillator as well as a light guide, and has been employed primarily in medical and non-destructive imaging as x-ray scintillators.
Scintillating fiber optic plates can be made thick in order to provide up to 100% radiation absorption efficiency without significant degradation in spatial resolution. This occurs because of the non-particle, low scatter, channeling nature of this type of luminescent device. While conventional scintillating fiber optic plates provide good radiation absorption efficiency and high spatial resolution, it would be desirable to be able to improve the brightness of such luminescent devices.
This goal was achieved in abandoned application Ser. No. 08/365,443 which discloses a hybrid luminescent device for imaging penetrating radiation that combines the brightness or strong luminescence output under penetrating or ionizing radiation and good spatial resolution capabilities of a thin phosphor screen and the radiation absorption efficiency and high spatial resolution of a thick screen, fiber optic scintillator.
A thin intensifying screen or phosphor layer typically has a higher index of refraction than a scintillating fiber optic face plate. Due to the differences in the index of refraction of these two luminescent devices, light refraction light scattering and back reflection of light will occur at the interface between these two luminescent devices. This results in an overall degradation of contrast sensitivity and resolution. This problem is made worse if there is any air gap present at this interface since air has a much higher index of refraction then either of the luminescent devices. Accordingly, it would be desirable to couple the two luminescent devices in an air free manner. It would also be desirable to be able to closely match the refractive indexes of the respective luminescent devices along their common interface.